1. Technical Field
The present invention relates to improvements to hydromechanical drawing machines and techniques for forming sheet metal blanks.
2. Background Art
Hydromechanical drawing is a process of forming sheet metal by clamping the edges of a sheet metal blank and drawing the central portion of the blank with a punch. The area below the blank is filled with a liquid, such as water. The liquid forms the blank against the punch surface. Liquid below the clamped edge of the blank lifts a portion of the blank where the blank enters the die cavity. The liquid below the blank at the flange reduces friction as the blank enters the die cavity.
One of the principal advantages of hydromechanical forming is that a second die surface may be eliminated in some applications with the liquid providing the reaction surface for the punch. In production parts, the required pressure is dictated by the tightest local radius to be formed. Maximum pressure must be applied to the entire surface of the blank. As a result, large presses must be used to perform the forming operation.
To improve the sharpness or part shape definition achieved in a hydromechanical forming operation, a second die may be incorporated in a hydromechanical forming tool. The second or lower die is only contacted after the drawing operation is nearly complete. Forming areas having a tight radius and other local features is completed by the punch driving the blank into engagement with the second die. The hydromechanical drawing process enables deep drawing of blanks provided that the maximum elongation of the blank is within the conventional forming limit diagram of the material being formed. Press size may be reduced by providing a second die because fine details may be formed when the punch engages the lower die.
One disadvantage with this approach includes the cost of providing a second die. Another disadvantage is that the punch and lower die must be precisely aligned to minimize die marks and surface imperfections. A further disadvantage is that the frictional force applied to the blank from both sides of the tool results in less uniform strain distribution. Another disadvantage of this process is that high volumes of liquid must be pumped into and out of the lower die cavity with each stroke of the press. Pumping large volumes of liquid in and out of the die takes a substantial amount of time and energy.
While it is not essential that the blank is supported on the liquid surface throughout the entire punch drawing operation, one of the principal advantages of hydromechanical forming is that liquid may be used to reduce friction at the upper perimeter of the lower die where the blank is drawn into the lower die.
Increasingly, new forming techniques are being developed for forming advanced high strength steel (AHSS), ultra high strength steels (UHSS) and specialized aluminum alloys that are difficult to form. One process that has been suggested to improve formability of such material is electro-hydraulic forming. However, deep drawing with electro-hydraulic forming is difficult due to the fact that the pressure exerted on the blank is very substantially reduced as the distance between the electrodes and forming surface increases. Similarly, as the volume of the electro-hydraulic forming chamber increases, the pressure available for forming the blank decreases. In forming shallow parts from such advanced materials, the distance from the electrode does not create a major issue. The reduction of pressure, as the blank is moved away from the electrode, reduces the ability of the system to deep draw a blank.
Applicant's invention is directed to solving the above problems and other problems as summarized below.